The complexity of present day logic circuits, for example those used in modern computer systems, has made trouble shooting of a malfunctioning circuit a formidable task. Isolating faults without the aid of interactive logic circuit testers, for example backtracing with a conventional oscilloscope, can consume an extraordinary number of man-hours and contribute to excessive computer system downtime.
Logic circuit testers typically include a probe of convenient size and weight for the user to make contact with various points on the circuit board under test. The circuit node to which the probe is directed is determined by the user by referring to a circuit diagram or may be determined by referring to the readout of a computer which analyzes the responses at the preceding node to determine where the probe should next be connected. Because of the switching speeds of modern logic circuitry, such probes must have a very high frequency response. They are, therefore, sensitive to high frequency noise which, if allowed to enter the tester system, may provide the user with misleading test results. One simple solution to avoid the effects of high frequency noise is to insert a series low pass filter to remove such noise. However, such filters affect the system response to high frequency data as well, being unable to discriminate between high frequency noise and high frequency data. The preferred embodiment of the present invention, therefore, incorporates pulse discriminating circuitry which rejects all pulses of width less than a predetermined time and accepts all pulses of widths greater than that predetermined time, irrespective of amplitude.
The pulse discriminator circuit comprises two paths, one for negative going and the other for positive going input pulses. Each circuit path accepts the input signal, whether data or noise, which simultaneously is applied to a one-shot multivibrator (one-shot) and to a time delay circuit in parallel with the one-shot, the outputs of which are applied to the inputs of a NAND gate. The NAND gate inhibits further signal processing unless the signals that are applied to its inputs are coincident. The output of the NAND gate is applied to the input of a flip-flop which is clocked by the output of the one-shot. The output of the flip-flop and the output of a corresponding flip-flop of the other path is applied to an output NAND gate. The output of the output NAND gate will not respond to input pulses of duration shorter than the predetermined time unless both flip-flops are set.
A logic tester system can also receive noise pulses when the probe used to contact a circuit node is not actually making adequate electrical contact with that node. Since the noise pulses received during misprobe contact with a node could exceed the predetermined width, misleading test results could again be produced and a good board could be rejected, a bad board accepted or the board may have to be retested. Misprobe contacts commonly occur when testing boards have oxidized conformal coating or other clear dielectric coatings covering the circuitry. To reduce the amount of retest necessary by assuring that adequate electrical contact is made, the preferred embodiment of the present invention incorporates a misprobe detection capability which detects an open circuit at the probe tip. The misprobe detection capability comprises a high impedance operational amplifier for receiving the output or the probe and comparing it to a reference voltage. The output of the operational amplifier is then applied to one input of a dual comparator circuit and the reference voltage is applied to the other input of the dual comparator circuit. If both outputs of the dual comparator are high, the probe is not making proper contact with the node under test and a misprobe indication is provided to the user .